In a thermal power plant including a steam turbine, technology to suppress emissions of carbon oxide is gaining attention in view of preserving global environment, and there are increasing needs to enhance efficiency of power generation.
To enhance power generating efficiency of a steam turbine, it is effective to increase a turbine steam temperature. In recent thermal power plants equipped with a steam turbine, a steam temperature is increased to equal to or higher than 600° C., and the temperature is showing a tendency to be increased to 650° C., and further to 700° C. in the future.
Around a turbine casing, a valve casing, a nozzle box, and a pipe, of the steam turbine, which is exposed to high temperature steam, high temperature steam circulates and increases their temperatures, and high stresses occur. Accordingly, the above are required to endure a high temperature and a high stress, and as a material constituting the above, there is demanded a material having superior strength, ductility, and toughness in a scope ranging from a room temperature to a high temperature.
Particularly when the steam temperature is over 700° C., a conventional iron-based material is insufficient in terms of high temperature strength, and therefore application of an Ni (nickel)-based alloy is considered. Due to its superior high temperature strength property and corrosion resistance, the Ni-based alloys have been widely applied mainly as materials for jet engines and gas turbines. As typical examples, Inconel 617 alloy (manufactured by Special Metals Corporation) and Inconel 706 alloy (manufactured by Special Metals Corporation) are used.
To enhance high temperature strength of an Ni-based alloy, there is a method in which either one of precipitated phases of a precipitated phase called a γ′ (gamma prime: Ni3(Al, Ti)) phase or a precipitated phase γ″ (gamma double prime: Ni3Nb) phase is precipitated or both the phases are precipitated in a parent phase material of the Ni-based alloy by adding Al and Ti. As what secures the high temperature strength by precipitating both the precipitated phases of the γ′ (Ni3(Al, Ti)) phase and the precipitated phase γ″ (Ni3Nb) phase, the Inconel 706 alloy, for example, can be cited.
On the other hand, there is also an Ni-based alloy in which Co (cobalt) and Mo (molybdenum) are added to strengthen (solid solution strengthening) a parent phase of an Ni base to ensure high temperature strength, such as Inconel 617 alloy.
As described above, as a material for a component of steam turbine for steam over 700° C., application of an Ni-based alloy is considered. It is demanded to improve high temperature strength of the Ni-based alloy by composition improvement or the like, while maintaining castability of the Ni-based alloy.